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DME Relay Trouble Shooting and Replacement
 
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Pelican Technical Article:

DME Relay Trouble Shooting and Replacement

Steve Vernon

Time:

1 hour1 hr

Tab:

$15 to $100

Talent:

**

Tools:

Multi-meter, wire, male spade connectors, 12volt battery for testing

Applicable Models:

Porsche 993 Carrera (1995-98)
Porsche 993 Carrera 4 (1995-98)
Porsche 993 Carrera 4S (1996-98)
Porsche 993 Carrera S (1998)
Porsche 993 Targa (1996-98)
Porsche 993 Turbo (1996-97)

Parts Required:

DME relay

Hot Tip:

Ground connections can be trouble

Performance Gain:

Running motor

Complementary Modification:

Check all fuses

If your Porsche 993 will not start or suddenly dies on you, one of the first places you should check is the DME relay. The relay powers both the DME and fuel pump on the 993 so if you are having fuel pump electrical issues you will need to check both the relay and the fuel pump fuse. These relays are prone to failure; a lot of failures. They usually fail from broken joints in the circuits but they can also fail from the diode and or have issues with heat sink.

This article will show you how to check your relay for proper function and show you an emergency stop gap to get you home, but this stop gap should only be used in the most extreme of emergency situations. If you are going to drive a 993, invest in a couple of extra DME relays and always carry a spare, there is even room in the fuse box for the spare. With a proper functioning relay buy a spare and swap it out with the good relay, if the car starts and runs with no problems then you know the new relay is good, switch them back and place the spare in the fuse box. While all of these tests will help track down a problem it is still easier just to install a known good relay

Before you start chasing any electrical problems always check that your battery is fully charged and in good condition.
Figure 1

Before you start chasing any electrical problems always check that your battery is fully charged and in good condition. Place your probes for the multi meter on the positive post (red arrow) and negative post (yellow arrow) of you battery. You should have between 12.2 and 12.7 volts. This battery is a little on the low side but acceptable.

Pull back the carpet in the front by the right side A-pillar and release the two clips on the fuse/relay panel cover (red arrows) and remove it from the vehicle.
Figure 2

Pull back the carpet in the front by the right side A-pillar and release the two clips on the fuse/relay panel cover (red arrows) and remove it from the vehicle.

Check the fuse and relay list and diagram located in your owner's manual and remove the DME relay (red arrow).
Figure 3

Check the fuse and relay list and diagram located in your owner's manual and remove the DME relay (red arrow). Gently wiggle and pull the relay out from its location in the panel, these fit snuggly and if you just grab the relay and pull on it you may just end up pulling the cover off the relay. Disconnecting power to the DME will cause it to erase its adaptive memory and you will need to drive the car for approximately ten minutes after repowering the DME to restore normal performance

On the underside of the relay will be the terminal numbers.
Figure 4

On the underside of the relay will be the terminal numbers. The schematics are as follows; 87b - power to fuel pump, 85b-ground from DME to pick up fuel pump, 87-power to DME and fuel injectors, 85- ground via DME, 86-ignition switch and 30 is battery or power.

If you need to run the fuel pump for testing purposes without running the motor or turning
Figure 5

If you need to run the fuel pump for testing purposes without running the motor or turning on the ignition key you can use a 15 amp fused jumper wire and install the leads in terminals 30 (red arrow) and 87b (yellow arrow)

To trouble shoot problems with the relay and wiring you can use these tests.
Figure 6

To trouble shoot problems with the relay and wiring you can use these tests. With the key off check for voltage at terminal 30 (red arrow)

With the ignition in the ON position check for battery voltage at terminal 86 (red arrow).
Figure 7

With the ignition in the ON position check for battery voltage at terminal 86 (red arrow).

Check for ground at terminal 85 (yellow arrow), if there is no ground then you should check the ground connection on the number one intake runner.
Figure 8

Check for ground at terminal 85 (yellow arrow), if there is no ground then you should check the ground connection on the number one intake runner. If you have ground, reinstall the relay with the ignition off. Place a wire under terminal 87 so you can get your multi-meter on it and turn the ignition switch to the ON position. If there is voltage then the relay is functioning correctly.

To test the relay itself, place your multi-meter in the position to test ohms or resistance.
Figure 9

To test the relay itself, place your multi-meter in the position to test ohms or resistance. You are going to be testing the resistance across terminals to determine if the relay is functioning correctly. Begin by placing the probes on terminal 30 (green arrow) and 87b (yellow arrow), the resistance should be infinite or a reading of 1 or OL depending on your meter. If this is different reading the relay contact points are not open and the relay is no good.

Next place your probe on terminal 30 (green arrow) and terminal 87 (yellow arrow), the resistance should be infinite or a reading of 1 or OL depending on your meter.
Figure 10

Next place your probe on terminal 30 (green arrow) and terminal 87 (yellow arrow), the resistance should be infinite or a reading of 1 or OL depending on your meter. If this is different reading the relay contact points are not open and the relay is no good.

If the readings are correct so far you are going to need to apply a power source to the relay, the car battery will do.
Figure 11

If the readings are correct so far you are going to need to apply a power source to the relay, the car battery will do. Connect the power source across terminals 85b and 87 (red arrows) and take a resistance reading across terminal 30 (yellow arrow) and 87b (red arrow). The reading should be almost zero or between 0.1 and 0.3 ohms

Connect the positive side of the power supply to terminal 86 (blue arrow) first and then the negative side to terminal 85 (green arrow).
Figure 12

Connect the positive side of the power supply to terminal 86 (blue arrow) first and then the negative side to terminal 85 (green arrow). Because of the diode in the circuit, if you install them the other way the electricity will not flow and the relay will not pick up. Place the probes on terminal 30 (yellow arrow) and terminal 87 (red arrow). The resistance should be close to zero again. If any of these tests fail the relay is bad and should be replaced. If the relay is good then you problem lies somewhere else.

In an absolute road side emergency you can wrap a paper clip around terminals 87, 87b and 30 and reinsert the relay long enough to get the car to a safe location.
Figure 13

In an absolute road side emergency you can wrap a paper clip around terminals 87, 87b and 30 and reinsert the relay long enough to get the car to a safe location. This is NOT a substitute for a proper relay and if you have a paper clip on you then you should have had an extra relay or two in the fuse box.

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